1. Field of the Invention
This invention relates to a performance improvement in screen printing apparatus.
2. Description of the Prior Art
In screen printing, a fine-meshed web 1 stretched in a taut state on a wooden or metal frame 4 is formed into a box-like screen, as depicted in FIG. 1. The desirable pattern is applied directly to the screen mesh by painting it with an impermeable medium. The screen stencil is then placed over the surface of a substrate to be printed, and ink is applied on the screen stencil by means of a squeegee 2. When the squeegee 2 slides across the face of the screen stencil, from one end to the other end, some of the ink goes through the permeable portions of the stencil where the medium is not painted, and is printed onto the substrate below the stencil. The impressions thus produced are an inversed copy of the original pattern.
The screen printing process is well known to be very versatile with a variety of inks. In addition, since the pattern printed comes in thick film ink, the application of a screen printing process is advantageous where impressions produced are required to have increased anti-weathering characteristics and resistance to chemicals. This printing method has to date been universally used in productions of printed circuit (PC) boards and integrated circuit (IC) boards, etc.
Referring to FIGS. 2(a) and 2(b), which illustrate examples of conventional screen printing, FIG. 2(a) is a side cross-sectional view of a frame 4 with a patterned mask on a screen mesh secured in a taut condition to the four sides of the frame. When a squeegee 2 is not pressed down, the stencil lies substantially parallel to the plane of the surface 10 to be printed, on which the mask is placed slightly spaced apart. When a squeegee 2 sweeps across the top face of the ink from one end to the other end, the stencil comes down onto the surface 10, by the pressure of the squeegee. In fact, with the sliding movement of the squeegee 2, the stencil bends into an inverted triangle, as shown in FIG. 2(b), with its apex moving across the surface 10. Since the stencil has its opposite ends held in fixed positions, the stencil in printing operation must have a physical stretch to a greater length than its original length which the stencil would have when not in printing operation by the pressure of a squeegee 2, as in FIG. 2(a).
It is well known to those versed in the art that the amount of this elongation is the smallest when the moving squeegee 2 passes the mid-point of a stencil in the printing operation. The amount of elongation increases more as the squeegee 2 slides farther away, in either direction, from that mid-point of a stencil. This phenomenon can easily be verified mathematically by the sine theorem.
A number of problems are due to the above-mentioned behavior of the stencil in the prior art screen printing process. With the elongation of the screen, the patterns formed in the stencil undergo a corresponding stretching. As a result, the images printed on the surface 10 have greater dimension than the original which the screen stencil would provide if the screen stencil were free from the pressure of the squeegee 2. In addition, the elongation in the images naturally becomes greater toward either end of the image.
In the prior art, the above-mentioned difficulties also involve another problem. When the screen stencil in a printing operation undergoes an increasing rate of elongation from the center to the ends thereof, the tension change which the squeegee 2 develops on the screen stencil also occurs in a correspondingly increasing amount. As a result, the holes in the mesh of the web screen become larger at both screen ends where screen tension reaches a maximum. Thus, more ink goes through the screen onto the surface 10 near its ends than near its center because of the enlarged holes in the mesh. Accordingly, the image is printed with larger dots of ink at its ends than at its center. Furthermore, the disproportionate amount of tension developed on a screen stencil with the sweep of the squeegee results in the shorter service life of the stencil.
In the prior art screen printing, the images obtained have disproportionate deviations from the pattern intended.
Due to the varying amount of tension developed on the screen stencil caused by the squeegee movement, the screen stencil has a shorter life. In addition, the differently expanded holes in the screen mesh produce a poor image, lacking in print quality.
It will be appreciated that, in FIG. 2(b), the snap-off angle .alpha. is the angle which the screen forms with the printed surface on the positive side of the ink held on the squeegee. This angle varies with movement of the squeegee, but such a variance of the snap-off angle is undesirable for keeping the distribution and thickness of the ink constant on the surface of the material to be printed.
U.S. Pat. No. 4,193,344 to Ericsson discloses a fixed squeegee and a movable stencil, respectively, in FIGS. 9-13 thereof, while FIGS. 1-8 thereof disclose a movable squeegee and a fixed stencil. Specifically, in the embodiment of FIGS. 9-12 thereof, a squeegee comprising a frame 91 having bearing means which carry a reciprocable drum 92 is fixed, that is, cannot move forward and backward (rightward and leftward thereof). Thus, in the embodiment of FIGS. 9-12 of Ericsson, it is impossible for any end of stencil 94 to move forward and backward in accordance with forward and backward movement of the squeegee. On the other hand, in the embodiment of FIGS. 1-8 thereof, frame 2, which corresponds to the end of the screen stencil, is rotated about the pivot axis 2a. However, frame 2 cannot move forward or backward. Thus, it is impossible for frame 2 to move forward and backward in accordance with movement of squeegee 4.